It is well-known to prepare tertiary alkyl ethers by reacting an iso-olefin generally contained in a hydrocarbon fraction with an aliphatic alcohol, generally used in excess, in the presence of an acid catalyst, for example sulfuric acid, hydrofluoric acid, aluminium chloride or boron fluoride, or int he presence of carbonaceous matter containing sulfonic groups (--SO3H), for example sulfonated coal, sulfonated phenol-formaldehyde resins, sulfonated coumarone-indene polymers or preferably sulfonated styrene-divinylbenzene copolymer resins or other compounds, notably mineral compounds, comprising sulfonic groups (for example sulfonated polysiloxanes).
It has been known for a long time that the reaction between an aliphatic monoalcohol and a tertiary olefin is balanced and that it is difficult to obtain iso-olefin conversion coefficients with a high purity and yield. Conventional processes such as those described for example in Hydrocarbon Technology International, Autumn 95, p. 21-27, comprise one or more reactor(s) for etherification to tertiary alkyl ethers followed by at least one fractionating zone, generally a distillation zone, whose bottom product is ether containing the lowest possible amount of monoalcohol(s).
This is the reason why the prior art recommended, in order to improve the performances of this synthesis, to add a complementary reaction section to the main reactor, as described for example in U.S. Pat. No. 5,364,975 in the name of the applicant. In this patent, the complementary reaction section is included, according to a preferred embodiment, in the reflux device of the fractionating section. It has also been proposed, for example in U.S. Pat. No. 4,503,265 or in patent applications WO-A-93/19,031 and WO-A-93/19,032, to draw off a product from an intermediate tray of the fractionating section, to feed this product into a complementary reaction section and to feed the product from this complementary reaction section back into the fractionating section at a level below the draw-off level. The drawback of this embodiment is that it disrupts the smooth running of the distillation process in the fractionating section. A process of the UOP Company known as Ethermax, wherein the effluent from the main etherification section is fed into a distillation-reaction zone, has also been described for example in Hydrocarbon Processing, March 1995, p. 114. As shown hereafter in a comparative example, this process has the drawback of requiring a column of very great height to obtain a very appreciable improvement of the global performances. These conventional processes for preparing tertiary alkyl ethers will be described hereafter in connection with FIGS. 1, 2 and 3.
One of the objects of the invention is to overcome the main drawbacks of the processes described in the prior art and to propose several ether synthesis embodiments allowing to maximize global conversion of the iso-olefins contained in hydrocarbon cuts.
The feed consisting of a mixture of C4, C5, C6 or C7 hydrocarbons comprising iso-olefins and at least one aliphatic monoalcohol generally used in excess is fed into the main reaction section represented by reactor R1 in FIGS. 1 to 9. The mixed reactants are brought into contact with an acid catalyst.
The product from this reaction section R1 is fed into a distillation zone represented by column F1 in FIGS. 1 and 2 and by column F2 in FIG. 3. It is distilled in this column in order to produce, at the bottom, through line 9, a tertiary alkyl ether containing the lowest possible amount of monoalcohol(s), and at the top, through line 2, a mixture of reactive and non reactive hydrocarbons and of aliphatic monoalcohol(s) carried over by azeotropy. This effluent flowing out through line 2 is condensed in condenser E1 and collected through line 3 in drum B1 prior to flowing into pump P1 through line 4.
In the instance schematized in FIG. 1 (U.S. Pat. No. 5,364,975), part of the effluent leaving pump P1 is fed through line 10 into a reactor R2 referred to as finishing reactor, whose effluent is fed through line 12 into column F1 as reflux and the rest of the effluent flows off as distillate through line 6.
In the embodiment schematized in FIG. 2 (U.S. Pat. No. 4,503,265), a product is drawn off from an intermediate tray of the distillation column between the effluent introduction point of etherification reactor R1 and the top of this column, and this product is fed through line 10 into a reactor R2 referred to as finishing reactor, whose effluent is fed through line 12 into column F1 at a level below the level of the draw-off point. In the instance schematized in FIG. 2, part of the effluent flowing out of pump P1 is fed through line 5 into column F1 as reflux and the rest of the effluent flows off as distillate through line 6.
In the embodiment schematized in FIG. 3 (Hydrocarbon Processing, March 1995, p. 114), the effluent of the reaction section R1 is fed into a distillation-reaction column beneath the first catalyst bed and part of the effluent flowing out of pump P1 is fed through line 5 into column F1 as reflux and the rest of the effluent flows off as distillate through line 6.
However, such a process requires in the instance schematized in FIG. 3 a great number of reaction zones in distillation-reaction column F2 in order to obtain high conversions into iso-olefins and consequently a column of great height. In fact, obtaining high conversions requires a distillation-reaction column containing many reaction zones. Insofar as one of these zones alone occupies about 3 meters in height in the distillation-reaction column, the final height of this column quickly becomes limitative. It would therefore be interesting to have such a high conversion to iso-olefins while keeping a limited number of reaction zones in the column and/or to limit the height of the column. This is one of the objectives of the invention as described hereafter in connection with FIGS. 4, 5, 6, 7, 8 and 9.